How Motors Work

Single loop motor armature. Image author Lookang, CC BY-SA 3.0 via Wikimedia Commons

These are quite good graphics, (not the animation above) on the Georgia State University site showing the BIL force on a conductor in a magnetic field, and how a traditional DC motor works. The first practical electric motor dates from 1834.

If I is the current flowing through the armature (the thing that turns in a motor) in amps, B is the magnetic flux density in tesla (how concentrated the magnetic field is) and L is the length of the armature in metres, then the force on the armature coil in newtons is:

 

F =BIL

 

One side of the coil experiences a force F in one direction and the other side a force F in the other direction (because the current flows the other way). These two forces are known as a couple. A couple creates a torque (twisting force) and the magnitude of the torque in newton-metres (units Nm which you'll sometimes see on cordless drill specs) is:

 

T = FW

 

where W is the width of the armature.

The effect of the torque is to turn the armature which is made up of coils wound around a laminated soft iron core, mounted on a shaft.

The motor in the graphic has a single coil. Real world universal motors (which run on AC or DC) in a corded power drill, vacuum cleaner, food mixer etc have an armature with many loops of wire so the electromagnets (the grey things in the graphic) can keep exerting a force as the armature turns. A commutator switches power to the loops via graphite brushes while the armature is turning.

Motors in electric vehicles and cordless tools work on a different principle. They don't use brushes and rely on the interaction between varying magnetic fields in both a stator (a fixed coil located around the internal perimeter of the motor) and a rotor which turns.